K Language: Remarks on Style (1995) [pdf]

1010data indeed still uses K as a core language for its infrastructure, supplemented with some C libraries here and there. The internally-maintained fork of k3 has grown some interesting features over the years, both unique and borrowed from newer K releases. I hear "Shakti" aka k7/k9 is used by a handful of cryptocoin exchanges. Their needs drove a number of design considerations in early k7, but I'm not up to date on k9. Most of the k4/kdb+/Q users I know work at hedge funds.

Well, one could certainly use a k (kdb) to solve the problem discussed by another item currently on the HN front page, which seems applicable to wide variety of industries: https://news.ycombinator.com/item?id=21923911

I’ve never used pure K, but Q is a real beauty. Maybe one of the best language I’ve used (though I haven’t used it in prod, so take what I say with a grain of salt).

But to answe your question, K/Q is mostly used at banks and some HFT operations.

long names obscure the patterns in expressions, and make idioms harder to pick out. In APL-family languages, the ability to visually pattern-match aids comprehension. It's a skill that must be learned, though. This may sound implausible until you've tried it.